Mail user authentication failure

My site has been suffering from a problem with access from Macs running Mail to POP3 mailboxes hosted on Snow Leopard server mail from at least 10.6.3 upwards (including the latest build of 10.6.5). It manifests itself as the notorious failed connections with Snow Leopard Mail, but the cause appears fairly clear.

If I look at the Mail Server logs (Mail Access) I can clearly see that the connection was rejected with the error message "unable to lookup user record ". If I then look at the Password Service Server Log it is clear that this server did not receive the authentication request, as it is not present in the log even though many successful authentications both prior to and subsequent to the failed one are present.

So somewhere between the Mail Server and the Password Server some authentication requests are going astray. It is only a very occasional occurrence but it appears to be totally random in nature - authentication will carry on correctly for hours and sometimes days, but then all of a sudden an authentication request will fail and Mail trips out on the client system. Once you reset Mail things again proceed fine but it is a nuisance that this happens at all.

I would like to see Apple address this in one of two ways - either sort out why the occasional authentication request fails, or alternatively make Mail not be quite so pedantic. If a connection fails then tolerate it - this does happen occasionally, for many different reasons, and it is a big nuisance having to calm Mail down when it does. Why not just have an error window like Entourage which you can look at if you want to see when errors have occurred?

In the meantime, if anyone has any good ideas about why the authentication requests fail on occasion I would be delighted to hear. This didn't happen at all originally for many iterations of the server software until suddenly it did start occurring, so it must be possible to make it work reliably!